In the present state of the art, triorganosiloxy-stopped polydiorganosiloxane fluids are produced by the reaction of cyclic diorganosiloxanes, linear silanol-stopped polydiorganosiloxanes or mixtures of cyclic diorganosiloxanes and linear silanol-stopped polydiorganosiloxanes with some appropriate chainstopper such as a hexaorganodisiloxane in the presence of acidic or basic equilibration catalysts (where the term equilibration is used to describe the phenomenon which exists when the ratio of linear organosiloxane polymers to cyclic organosiloxane oligomers is brought to and maintained at a constant value). See, for example (a) "The Siloxane Bond" Eds.: Voronkov, M. G.; Mileshkevich, V. P.: Yuzhelevskii, Yu. A. Consultant Bureau, New York and London, 1978; and (b) Noll, "The Chemistry and Technology of Silicones", Academia Press, New York, (1968). Such reactions result in mixtures of (primarily) triorganosiloxy-stopped linear diorganosiloxanes and cyclic diorganosiloxanes. A typical example is the production of trimethylsiloxy-stopped dimethylsilicone fluids by the acid or base catalyzed equilibrations of mixtures of cyclic dimethyl siloxanes, silanol-stopped polydimethylsiloxanes and (preferably) hexamethyldisiloxane. Efficient silanol condensation and water removal is necessary in such reactions to produce low silanol products. Several patents, e.g., (a) Siciliano, George R.: U.S. Pat. No. 3,853,933, which describes the use of two fixed beds packed with acid treated carbon black under vacuum to equilibrate siloxanes; and (b) Ottinger, S., et al. U.S. Pat. No. 4,792,596. describe the use of fixed catalyst beds to effect siloxane polymerization. In a recent patent, Elms, R. A., U.S. Pat. No. 4,831,174, are described reaction mixtures high in silanol content which were rapidly equilibrated (30 minutes residence time or less) at 100.degree. to 150.degree. C. by passing inputs through 2 fixed beds of acid treated clay under pressures from 5 to 1520 torr. The resulting equilibrates were low in silanol content (from about 100 to 300 ppm Si--OH as listed in the examples). Finally, a series of Wacker Chemie patents, e.g., U.S. Pat. Nos. 2,830,967 (1958); 3,186,967 (1965); 3,706,775 (1972); 4,725,643 (1988); and 4,975,510 (1990) disclose the use of a variety of organonitrogen derivatives of phosphorus or phosphoric acid or linear phosphonitrilic chlorides for the efficient condensation of silanols.
Recently, it has been found that with certain catalysts, such as phosphonitrilic chlorides, that two linear siloxanes such as MD(x)M (where M is a trimethylsiloxy group; D is a dimethylsiloxy group and MD(x)M is a trimethylsiloxy-terminated polydimethylsiloxane of x units) and MD(y)M may be disproportionated to form a new MD(z)M, wherein z lies between x and y. It has further been discovered that cyclics/linears equilibration is inefficient under the reaction conditions; i.e., cyclic siloxanes are slow to react and cyclic siloxane formation from linears is also slow.
It would be advantageous for a process if cyclic siloxanes could be used in combination with linear (primarily silanol-stopped) polysiloxanes and a source of triorganosiloxy chainstopper (in the case of Polydimethylsiloxane fluids, preferably hexamethyldisiloxane, MM) reacting to rapidly yield a mixture of triorganosiloxy-stopped polydialkylsiloxanes with, preferably, no more than 16 weight percent cyclic siloxanes. It would be especially preferred that the linear species resulting from these processes have low branching (trifunctional)- and low silanol contents, as well. The resulting mixtures would subsequently be stripped to produce fluids having viscosities primarily in the range from about 10 to 1000 cps. Volatile siloxanes vaporized off during any process step could be collected and recycled as starting material for future fluid syntheses.
Such objectives are achieved by the two step process of the present invention which combines the silanol condensation and siloxane disproportionation capabilities of catalysts like linear phosphonitrilic chlorides (LPNC's) with the cyclics/linears equilibration capabilities of fixed beds of catalysts such as acid-treated clays, and the like. The processes can be carried out in a variety of ways, as will be exemplified. For example, in one embodiment, a siloxane reaction mixture containing an appropriate chainstopper is sent through a fixed bed of catalyst, then the output treated with linear phosphonitrilic chloride to complete silanol condensation and siloxane disproportionation. Alternatively, the steps can be reversed, and the chainstopper can be added in either or both steps.
The process to be described hereinafter has, after numerous trials, been found to be operational with cycle times of as little as 25 minutes for lower viscosity fluids when a vacuum less than 10 torr is used. For higher viscosity fluids and pressures greater than 10 torr reactions are only somewhat longer (approx 40 to 45 min). Operating temperatures are as low as 90.degree. C. The resulting mixtures have as little as 200 ppm or less Si--OH content, by weight, most often less than 100 ppm.
Thus a process has now been found which rapidly and efficiently reduces the silanol content of the starting diorganopolysiloxane diols, and provides triorganosiloxy end groups, while utilizing cyclics via equilibration.